If there’s one thing I’ve learnt about image processing, it’s that there always something else to learn. On occasion, it’s worthwhile going back to some older images to see if they can be reprocessed to better effect. My original post on NGC 4038 was here: http://blog.andrewluck.me.uk/2009/02/28/the-antennae-galaxies. The widefield image suffered a lot from colour noise and this was due to the dark frames used not removing some of the brighter pixels and overcompensating for others resulting in black dots on the luminance frames and colour spots on the RGB frames.
When re-processing, I only calibrated the colour images, relying on the stacking process to average out the noise on the luminance. Then I manully removed the brightest colour dots which remained. Once the combine had been done I used GradientXterminator to remove a lot of the background noise.
